Mass loss during the drying process testing, commonly known as loss on drying (LOD) determination, is a fundamental gravimetric technique used to quantify the moisture and volatile content of polymers, rubber chemicals, pharmaceuticals, food products, and biomass materials. Accurate measurement of drying mass loss is essential for incoming material inspection, drying process validation, and compliance with international standards such as GB/T 6284, ASTM D6980, and ISO 15512. This article explains the testing principle, applicable sample types, standard test methods, and practical applications of mass loss on drying measurement.
Technical Overview
Determination of mass loss during the drying process, also referred to as loss on drying (LOD), drying loss, or moisture and volatile matter content, is a gravimetric analytical procedure in which a test specimen is weighed before and after drying under strictly controlled temperature, time, and atmosphere conditions. The reduction in mass observed after drying is attributed to the evaporation of free water, bound moisture, and low-boiling volatile substances from the sample matrix.
The test result is expressed as the percentage of mass lost relative to the original specimen mass, calculated using the following relationship:
Mass loss on drying (%) = [(m1 − m2) / m1] × 100
In this formula, m1 is the mass of the specimen before drying and m2 is the mass of the specimen after drying to constant weight. All weighings are performed on an analytical balance with a resolution of 0.1 mg, and specimens are cooled in a desiccator before each weighing to prevent moisture re-absorption from the ambient atmosphere.
It is important to distinguish mass loss on drying from true water content. LOD measures all substances that evaporate under the specified drying conditions, including water, residual solvents, plasticizer fragments, and other volatiles, whereas specific water determination methods such as Karl Fischer titration respond only to water. For this reason, drying loss values may be slightly higher than Karl Fischer results on the same material. The general procedure is standardized in documents such as GB/T 6284 for chemical products using the drying loss method, ASTM D6980 for moisture in plastics by loss in weight, ASTM D5668 for rubber chemicals by loss on heating, ASTM D2216 for soil and rock, ASTM D4442 for wood and wood-based materials, ISO 287 for paper and board, and ISO 15512 for the water content of plastics.
Testing Samples
Polymer Resins and Engineering Plastics
Hygroscopic resins such as PA6, PA66, PC, PET, PBT, ABS, PMMA, and TPU are routinely tested for drying mass loss. Excessive residual moisture in these materials causes hydrolytic degradation during melt processing, surface defects such as silver streaks and bubbles, and reduced mechanical strength in finished parts. Drying loss results are used to verify that hopper-dried material has reached the processing window specified by the resin manufacturer.
Rubber Materials and Rubber Chemicals
Raw rubbers, accelerators, antioxidants, processing aids, and carbon black are evaluated for loss on heating in accordance with methods such as ASTM D5668 and ASTM D1509. Moisture and volatile levels in rubber compounds influence scorch behavior, cure kinetics, porosity, and the dimensional stability of vulcanized products.
Fine Chemicals and Inorganic Powders
Pigments, dyes, fillers such as calcium carbonate and talc, salts, fertilizers, catalysts, and battery materials are tested to confirm that moisture content remains within specification. Accurate LOD data are essential for stoichiometric formulation, packaging in moisture-barrier containers, and calculation of active ingredient content on a dry basis.
Pharmaceutical Excipients and Food Powders
Starches, lactose, cellulose derivatives, milk powder, flour, protein isolates, and other nutritional or pharmaceutical powders are characterized for moisture content because water activity affects shelf life, flowability, caking tendency, compaction behavior, and microbiological stability.
Wood, Paper, and Biomass Materials
Wood-based panels, pulp and paper, natural fibers, and solid biomass fuels are tested for moisture content using oven-drying methods described in ASTM D4442, ASTM E1756, and ISO 287. Moisture level is a key quality and pricing parameter for bioenergy feedstocks and fiber-based products.
Testing Items
Typical test items reported under drying mass loss determination include the following:
- Moisture content determined by loss on drying at a specified temperature
- Loss on drying (LOD) or drying loss expressed as a mass percentage
- Volatile matter content released at elevated drying temperature
- Combined moisture and volatile matter content
- Residual moisture after drying, conditioning, or storage trials
- Moisture uptake kinetics through conditioned exposure studies
Results are reported with the drying temperature, drying duration or endpoint criterion, specimen mass, and number of replicate determinations, so that values can be compared against material datasheets and purchase specifications.
Testing Methods
Constant-Weight Oven Drying Method
The reference procedure uses a forced-ventilation or gravity-convection drying oven. A clean weighing bottle or dish is dried and tared, a homogenized specimen of typically 2 g to 10 g is added, and the container is placed in the oven at the specified temperature, commonly 105 °C ± 2 °C for general moisture determination or a product-specific temperature for thermally sensitive materials. Drying continues until consecutive weighings after cooling in a desiccator differ by no more than 0.3 mg or another defined constant-weight criterion. The mass difference relative to the initial specimen mass yields the drying loss percentage.
Rapid Moisture Analyzer Method
Halogen or infrared rapid moisture analyzers integrate a heating source and a precision balance in a single instrument. A 1 g to 10 g specimen is spread evenly on a filter pad, and the instrument heats the sample while continuously monitoring mass until an automatic shutoff criterion is reached, typically within 5 to 20 minutes. This approach is widely used for in-process quality control because of its speed, although results should be correlated with the reference oven method for each material.
Vacuum Oven Drying Method
For heat-sensitive substances such as certain pharmaceuticals, food powders, and polymer additives, drying is performed in a vacuum oven at reduced pressure and a moderate temperature, for example 60 °C to 70 °C. The lowered boiling point of water under vacuum allows efficient moisture removal without triggering thermal decomposition, oxidation, or loss of high-boiling components that would bias the result in a standard oven.
Thermogravimetric Analysis (TGA)
Thermogravimetric analysis records specimen mass continuously while temperature is ramped at a controlled rate using milligram-scale samples. The resulting mass-loss curve distinguishes the low-temperature moisture and volatile region from higher-temperature decomposition events, providing insight that is particularly valuable in research and failure analysis, for example when delamination or bubble defects in molded parts must be traced to residual volatiles.
Regardless of the selected method, good practice includes preconditioning of weighing vessels, homogenization and representative sampling of the specimen, protection of hygroscopic samples from atmospheric moisture during transfer, cooling in a desiccator before weighing, and duplicate determinations with agreement criteria defined in the applicable product standard.
Applications
Determination of drying mass loss supports a broad range of industrial and laboratory activities:
- Incoming inspection and supplier qualification of resins, chemicals, and additives against purchase specifications
- Validation and routine monitoring of drying equipment settings, including drying temperature, residence time, and dew point of the drying air
- Process window control for injection molding, extrusion, blow molding, and compounding of hygroscopic polymers, where residual moisture above critical thresholds causes hydrolysis and surface defects
- Batch release and certificate-of-analysis testing for chemical and pharmaceutical products
- Research and development studies on formulation stability, moisture sorption, and packaging performance
- Failure analysis of defective parts, coatings, or adhesives where entrapped volatiles are a suspected root cause
- Verification of dry-basis content in commercial transactions of powders, fibers, and biomass fuels
By quantifying how much mass a material loses under defined drying conditions, manufacturers can prevent processing defects, ensure consistent product quality, and demonstrate compliance with customer and regulatory requirements through accredited third-party laboratories or in-house quality laboratories.
FAQ
What is the difference between mass loss on drying and moisture content?
Mass loss on drying captures every component that volatilizes under the drying conditions, including water, residual solvents, and other low-boiling substances. Moisture content in the strict sense refers only to water, which is best measured by methods such as Karl Fischer titration. In most materials the two values are close, but for solvent-containing or additive-rich samples, LOD results may be higher.
What drying temperature and duration are typically used?
A drying temperature of 105 °C ± 2 °C to constant weight is the most common default for general moisture determination. Product standards may specify different conditions, for example 80 °C to 120 °C for various plastics, 125 °C for loss on heating of rubber chemicals, or 60 °C under vacuum for heat-sensitive pharmaceutical and food matrices. Duration depends on the specimen and typically ranges from one hour for thin powders to several hours for dense pellets.
What does "constant weight" mean in practice?
Constant weight is reached when the difference between two consecutive weighings, each performed after cooling the specimen in a desiccator, does not exceed a defined tolerance, commonly 0.3 mg for specimens below 5 g. Reaching constant weight confirms that further drying does not remove additional volatiles and that the measured mass loss is complete.
How much sample is required for a test?
A typical determination uses 2 g to 10 g of homogenized material for oven methods and 1 g to 10 g for rapid moisture analyzers. TGA requires only 5 mg to 50 mg. Larger specimens improve representativeness for heterogeneous materials, while smaller specimens dry faster and reduce the risk of crust formation that can trap moisture.
How are heat-sensitive samples tested?
Specimens that soften, melt, oxidize, or decompose at the standard drying temperature are tested in a vacuum oven at reduced pressure and lower temperature, or by an alternative technique such as Karl Fischer titration, distillation, or TGA, as specified in the relevant product standard.
Why did my molded parts show silver streaks despite passing a moisture test?
Silver streaking can occur when residual moisture exceeds the very low thresholds required by some resins, for example processing-grade PET, which may demand levels below 0.005%. Such low water contents are near or below the practical detection floor of gravimetric LOD methods, so methods with higher sensitivity for water, such as coulometric Karl Fischer titration in accordance with ISO 15512, are recommended for these materials.